Prior methods of making honeycomb core include laying a series of glue lines of a specific width and spacing on a sheet of desired substrate, to give a pre-glued sheet. The pre-glued sheets are then stacked with the glue lines offset from one sheet to another. The glue is activated under pressure and then the bonded stack is expanded or stretched to form the honeycomb. Typically, the expanded honeycomb block is then dipped one or more times to impregnate and/or coat the substrate with any of a variety of coating materials, such as a phenolic resin.
In an alternative version of this technique, corrugated material is used as the preferred substrate, with the glue lines applied to the high side of the waves of the corrugated sheet. The pre-glued corrugated sheets are staggered and the glue activated. The corrugated product is then dipped or coated as desired.
In another method, as described in U.S. Pat. No. 5,139,596, a honeycomb core is made one layer at a time. The honeycomb core is made by repeatedly passing an appropriately shaped roller, and forcing heated air, over a chosen substrate that is placed over a group of rods shaped to complement the roller. Upon passing the roller over the complementary-shaped rods, the roller and heated air cause the material to take the shape of the rods and the roller. Repeatedly passing the roller over new layers of rods and substrate forms the core one layer at a time.
These methods do not, however, provide for the formation of a full block of honeycomb core having well-defined cells in a single step while carefully controlling heat and pressure applied to the core. Further, these methods do not provide for the formation of a honeycomb core without using heated air or dipping, which can cause the release of noxious, and potentially toxic, fumes into the air, thereby creating a hazard for workers and pollution harmful to the environment.